The action of the human heart is controlled by propagation of electrical activity in various regions of the heart. The presence of abnormal accessory pathways in the heart can lead to conditions such as ventricular tachycardia and atrial flutter. These conditions are very common. Approximately 20% of the population will have some type of electrical disturbance activity in the heart during their lifetimes.
Physicians have found that they can detect malfunctions of the heart by probing the heart with a catheter fitted with one or more electrodes and having steering capability, measuring voltages within the heart, and observing the waveforms. Once a physician understands how the electrical activity of the heart is operating he can, if he wishes to do so, choose to "disconnect" certain portions of the heart electrically by the process of ablation. If multiple electrode are used, the catheter can make multiple readings simultaneously when it is curved inside the heart. Thus, the use of multiple electrodes shortens the time required to map the heart.
The electrical activity of the heart is detected and read in accordance with a mapping procedure to determine the presence of abnormal accessory pathways in the heart. A typical mapping procedure involves using electrophysiology sensing electrodes mounted on a catheter as remote-controlled voltage-testing probes to test various locations in the heart.
The process of ablation is a destructive process in which the catheter is used to burn a certain section of the heart which stops the propagation of an electrical signal from one portion of the heart to another. Alternate means to perform ablation have been to inject a chemical such as ethanol in specific regions of the heart, to apply very cold temperatures in a process called cryo-ablation, and to use sonic energy, which is sometimes referred to as ultrasonic ablation. The ablation process may alternatively consist of applying low-frequency RF energy to the heart tissue in order to create a burn. This burn will cause the tissue to heat up and desiccate and finally necrose.
Electrophysiology catheters are typically positioned at various positions in the heart under x-ray guidance. The x-rays show the catheter, and can also show the heart itself and thus the position of the catheter relative to the heart if dye injections are made. The clinician tries to visualize the position of the catheter in the heart in the various chambers. Electrical means are used to determine whether or not the electrode is in contact with the heart, and this information is shown on an EKG display. During the course of a typical procedure the operator will frequently return to one or more positions, and will look for particular waveforms that he sees from the sensing electrodes to determine whether the catheter has returned to the desired position. Typically, more than one catheter is used in a given procedure, and the catheters are constructed with steering or torquing devices that assist in positioning of the catheters within the heart.
The sensing or ablation electrodes of intracardiac catheters are typically made of tantalum, gold, or platinum. There can be as few as one or as many as five or more electrodes in a sensing and ablation catheter. Typical sensing and ablation catheters will have at least one tip electrode and two, three, or four ring electrodes proximal to the tip electrode. The proximal ring electrodes are typically spaced from the distal tip in two, three, or four-millimeter increments. The ring electrodes are generally bonded or crimped onto the catheter body or blended into the catheter body. The rings are sufficiently thick to have enough mechanical strength when crimped to adhere to the catheter shaft.
It is known that the injections of chemicals such as ethanol into the heart can produce a response which is similar to that produced when a burn is made in the heart. Basically, the injection of chemicals disrupts or cuts off electrical pathways in the heart by causing localized cell death.
The disorders that can be treated by ablating cardiac tissue include general arrhythmias, ventricular tachycardia, atrial fibrillation, atrial flutter, and Wolff-Parkinson-White Syndrome (WPW). Typically, ventricular tachycardia and WPW are treated by RF coagulation or DC discharge applied to cardiac tissue by electrode-tipped, deformable, and preset curved catheters. These catheters are of similar construction to those used in the art for electrically mapping the heart.
In order to navigate through the patient's vascular system, cardiac catheters are limited to small diameters. A typical mapping or ablation catheter has small electrodes mounted on the distal end of the catheter shaft. The electrodes can be arranged in bipolar pairs at the distal end of a catheter to ablate tissue by passing RF or DC electrical current between them through the surrounding myocardium. Alternatively, a single electrode could be disposed at the distal tip of a catheter, the single electrode being used to cause RF or DC electrical energy to pass directly through the heart tissue to a grounding plate on the surface of the patient's body.
Typically, the area of cardiac tissue that must be ablated is several times the size of the ablation region of the small electrode ablation catheters. Thus, a carpet bombing approach (i.e., ablating at many discrete sites) can be used to successfully treat cardiac disorders. This technique can lead to nonuniform ablation, as well as incomplete ablation if the ablation electrodes are not always directly in contact with myocardial tissue at each discrete site.
It is known to use a suction hole at a distal end of a catheter to engage tissue and thereby to hold the catheter in a fixed location in a patient's body while a distal ring electrode is placed in contact with tissue.
An alternative method for treating disorders in the heart is described in PCT application US93/09422, filed Oct. 4, 1993 by Daniel Bruce Fram et al. As described in that application, a catheter having a balloon mounted on its distal end is inserted into the coronary sinus or great cardiac vein. The balloon is inflated with fluid within the coronary sinus and is heated by a heating device located within the balloon. Tissue surrounding the coronary sinus is ablated by thermal conduction from the fluid to the tissue through the wall of the balloon.
Electrophysiological catheters can apply radio frequency energy to produce burn lesions at selected points in the heart to correct arrhythmias. By destroying the cells that constitute defective conductive pathways, the arrhythmias are stopped. Typically, rigid electrodes, of ring form either partially or totally surrounding the catheter shaft, are used, though it is desirable at times to produce larger lesions than can be produced with such electrodes. By using a larger electrode, one could apply higher power, and by spreading the current at conventional current intensity over a larger area, the larger lesion can be produced. The diameter of such conventional electrodes, however, has been limited by the size of access hole that can be tolerated in the artery. Also, the length of these electrodes has been limited by the need to maintain maneuverability for the catheter to pass through tight curves in proceeding through the arterial system and into the heart.